1. Technical Field of the Invention
The present invention relates to a continuous wave laser irradiation apparatus utilized for crystallizing a semiconductor film. In addition, the present invention relates to a method for manufacturing a semiconductor device including a process for crystallizing the semiconductor film with the use of the laser irradiation apparatus.
2. Description of the Related Art
A thin film transistor using a polycrystalline semiconductor film (polycrystalline TFT) is superior to TFT using an amorphous semiconductor film in its mobility by double digits or more and has an advantage that a pixel portion and its peripheral driver circuit in a semiconductor display device can be integrally formed on the same substrate.
The polycrystalline semiconductor film can be formed over an inexpensive glass substrate when a laser annealing method is employed. However, the energy of the laser light output from the oscillator fluctuates by at least a few percentage points due to the various reasons. This fluctuation prevents the semiconductor film from being crystallized homogeneously. When the crystallinity of the polycrystalline semiconductor film varies due to the inhomogeneous crystallization, the characteristic of TFT using the polycrystalline semiconductor film as its active layer such as on-current or the mobility also varies.
For example, in the case of an active matrix light-emitting device with a light-emitting element and a TFT for controlling current supplied to the light-emitting element provided in each pixel, when the on-current of TFT varies, the luminance of the light-emitting element also varies accordingly.
Moreover, when the semiconductor film is crystallized by the irradiation of the laser light in the atmosphere, the surface of the semiconductor film becomes somewhat rough. The higher the energy intensity of the laser light is, the rougher the surface of this semiconductor film becomes. The light is scattered to give more brightness in the region whose surface is rougher. Therefore, sometimes the striped light and shade are visible at intervals of several mm due to the energy fluctuation.
It is noted that the state of the surface of the semiconductor film is closely related to the oxygen in the atmosphere when the laser light is irradiated according to the patent application shown below.
Published patent application No. 2000-138180 (P.3-P.4) describes that the more oxygen the atmosphere contains, the rougher the surface of the semiconductor film crystallized with the irradiation of the laser light becomes. The application also describes to spray the semiconductor film with Ar when the laser light is irradiated.
When the surface of the semiconductor film becomes rough, interface state density at the interface between the semiconductor film and a gate insulating film formed so as to contact the semiconductor film becomes high and the threshold voltage shifts to normally-off side. Therefore, when the state of the surface of the semiconductor film becomes uneven due to the energy fluctuation of the laser light, the interface state density at the interface between the semiconductor film and the gate insulating film formed afterward varies, which results in the variation of the threshold of TFT.
When the laser light having high absorption coefficient to the semiconductor film is employed, it is possible to crystallize the semiconductor film more effectively. The absorption coefficient depends on the material and the thickness of the semiconductor film. However, When a silicon film having a thickness from several tens nm to several hundreds nm which is usually used in the semiconductor device is crystallized by an excimer laser or a YVO4 laser, the second harmonic having a shorter wavelength than the fundamental wave is higher in absorption coefficient and thereby it is possible to crystallize more effectively.
For this reason, in order to enhance the efficiency of the crystallization, the wavelength is usually converted through a non-linear optical element. The laser light converted into the harmonic, however, tends to have lower energy compared with the case of the fundamental wave. For example in the case of Nd:YAG laser, the conversion efficiency from the fundamental wave (wavelength: 1064 nm) to the second harmonic (wavelength: 532 nm) is approximately 50%. When the energy of the laser light decreases, the throughput in the crystallization also decreases, which results in the lowering of the productivity.
Moreover, since the non-linear optical element is easy to deteriorate due to the laser light and is inferior in endurance, when the energy of the fundamental wave is increased in order to obtain the laser light of the harmonic having high energy, it is necessary to do the maintenance frequently. Therefore, this is not preferable.
3. Problem Solved by the Invention
In view of the problem described above, it is an object of the present invention to provide a laser irradiation apparatus being able to suppress the unevenness of the crystallinity or the state of the surface of the semiconductor film and to perform homogeneous crystallization of the semiconductor film. It is another object of the present invention to provide a method for manufacturing a semiconductor device with the use of the laser irradiation apparatus being able to suppress the variation of the on-current, the mobility, and the threshold of TFT.
Furthermore, it is an object of the present invention to provide a laser irradiation method and a laser processing apparatus having high throughput in view of the problem described above.